I'm going to declare that the hack is closed or changed in firmware version 00.01.09.00.00
At least until the clever people can get hold of the firmware and take a look at it.
It is not possible to downgrade to 00.01.08.00.01 which is known to work and available at rigolna.
I've tried a 2gb and 8gb usb sticks formatted FAT32 with the magic bytes at the correct location. I've tried sending the the SCPI command by 2 different Windows packages and most recent using Python under linux. Code snippet below.
>>> rigol = rm.open_resource('USB0::6833::1603::DG8Axxxxxxxxx::0::INSTR') (xxxxxxxxx is serial no.)
>>> rigol.write(':DISP:BRIG 1')
(28, <StatusCode.success: 0>)
>>> rigol.write(':DISP:BRIG 100')
(28, <StatusCode.success: 0>)
Insert key, waiting dialogue appears, wait
>>> rigol.write(':PROJ:MODE DG992')
(32, <StatusCode.success: 0>)
Power off, remove key, power up.
The 2 DISP strings are just sent to confirm the DG811 is receiving commands by dimming then brightening the display. Then insert key, and after waiting for dialogue box to disappear, send PROJ command. Then power down and up.
Unfortunately, no upgrade.
Happy to help from here on.